Direct synthesis of indazole derivatives via Rh(III)-catalyzed C-H activation of phthalazinones and allenes

A novel Rh(III)-catalyzed annulation of phthalazinones or pyridazinones with various allenes was developed, leading to the formation of indazole derivatives bearing a quaternary carbon in moderate to good yields. The targeted products were synthesized sequential C-H activation and olefin insertion, followed by β-hydride elimination and intramolecular cyclization. The synthetic protocol proceeded efficiently with broad functional group tolerance, high atom efficiency and high -selectivity. The practicability of this method was proved by synthetic transformation.